Shadow Casting Principles

One of the most important concepts in dental radiography is the concept of shadow casting. Once the operator realizes the correlation between the position and angulation of the various elements in radiography and the way ordinary shadows are cast (say, the way your own shadow is cast on a wall or the ground on a sunny day) the entire process of film and source placement becomes easier to understand.

In this discussion, we will be dealing with four terms: Source, receptor, object and angulation. We will also be drawing analogies between the dental radiographic technique and an everyday example of casting shadows.

The radiographic source of light (x-rays) is the focal point in the x-ray tube. The receptor in radiographic technique is the film or the CCD of a digital radiographic sensor. The everyday source in our analogy is the sun, or a slide projector. The everyday receptor is the ground or a wall upon which a shadow is cast. The objects in dental radiography are oral structures such as bone and teeth, while in the everyday example, we will be using our own bodies as the object who's shadow is cast on the receptor.

Principle One

X-rays should be emitted from the smallest source of radiation possible. Large sources cause fuzzy images. As electrons strike the focal spot in the x-ray tube, X-rays are emitted. The smaller the focal spot the greater the detail. Manufacturers govern the size of the focal spot, and it cannot be changed by the operator. However, the focal spot can become enlarged over time due to continuous use. When focal spot enlargement does occur, the image becomes less sharp. The focal spot must be monitored through a quality assurance program. Resolution test devices will determine any change in the focal spot size.

In the everyday example, while the sun is a huge object, it's extreme distance from us makes it look very small in the sky, and the shadows it casts on a clear day are very sharp. Contrast this with a day in which the sun is obscured by a thin layer of clouds. The clouds have the effect of diffusing the sunlight over a much larger area making the light source much larger as well. If the cloud layer is thin, shadows will still be cast, but your shadow will lack all definition and sharp edges. As the cloud layer thickens the more the sun's light is diffused causing the shadow we cast on the ground to lose even more definition.

Principle Two:

The x-ray source-to-object-distance should be as long as possible. The use of a long-position indicating device (a long cone which is also lined with lead) will enable the x-ray photons to emerge in a straighter line; therefore producing a more accurate shadow. The straighter the x-ray photon line, the less divergent the beam. The resulting image will be a more accurate presentation of the radiographic structure.

In our everyday example, substitute a very bright light source, say a slide projector for the sun, and lets say that you are standing several feet from a wall. If the projector is located a long distance from you, your shadow on the wall will be a fairly accurate representation of your height and width. On the other hand, if someone moves the projector closer to you, your shadow is magnified in all directions and and is no longer representative of your height and width. The same thing happens in dental radiography, which is why all new machines come with long cones.

Principle Three:

The object-to-receptor-distance should be as short as possible. Placing the object close to the receptor reduces magnification and increases image sharpness.

In an everyday example quite familiar to most modern persons, consider that we are flying in an airplane at 10,000 feet on a sunny day. The shadow of the airplane on the ground may look quite sharp to us as we gaze down on it from on high, but to an observer on the ground, the shadow lacks sharp edges and is actually quite a bit larger than the actual size of the airplane itself. On the other hand, once the airplane lands, the shadow cast from the sun when it is directly overhead and unobstructed is almost the same exact size as the airplane itself, and the plane’s shadow has edges that are sharp.

Principle Four:

The receptor and long axis of the tooth should be parallel. When the receptor and the long axis of the tooth are parallel, as in the paralleling technique, the distortion of the recorded image is decreased.

In our everyday example, a projector casting our shadow on a perpendicular wall shows a reasonable representation of our shape in the shadow. On the other hand, we stand upright on the earth and as the sun sets, our shadow on the ground gets longer and longer. In addition, the elongation in the shadow is greater at the feet than at the head. Finally, if the sun is nearly directly overhead, our shadow will be extremely foreshortened. This sort of distortion is very important when taking periapical films, since there often is not enough room in the mouth to place the film exactly parallel to the teeth.

in the images below, the one on the left shows an extracted tooth lying flat on the film with the x-ray beam aimed at 90 degrees to both. It shows the truest representation of the tooth size and shape. In the x-ray on the right, the film and the beam are in ideal alignment, with the beam at 90 degrees to the film. However, the crown of the tooth was tilted up and lies at about 30 degrees to the film and beam. You can see that the tooth in this image is foreshortened. This image shows what happens in the all too familiar scenario in which a Rinn apparatus is used to keep the film and beam properly aligned while the apparatus itself is placed in the mouth at an angle to the teeth because there is not enough space in the palate or the floor of the mouth to align it properly. The best (and easiest) method of compensating for this condition is to use a technique which splits the angle between the film and the tooth.

Principle Five:

The x-ray beam should be perpendicular to the receptor. To achieve this principle, the x-ray beam must also be perpendicular to the long axis of the tooth. When this principle is not followed, the resultant image may appear either foreshortened or elongated. If the beam is at a lateral angle to a group of adjacent teeth, the crowns of the teeth may appear to be overlapped thus obscuring the contacts.

This is especially important when taking bitewing x-rays in which the contacts between the teeth must be clearly visible. Misangulation of the x-ray beam causes the shadows of the adjacent teeth to appear on the film to overlap obscuring incipient caries and other anatomical structures. This principle even applies to a single tooth when multiple structures, such as the nerve space and a filling may overlap in various ways depending on the relative angulations of the the source and the tooth.

The radiograph on the left was taken with all three elements, the film, the teeth, and the beam in optimum allignment. The film is parallel to the teeth, and the beam is perpendicular to both. Notice that the contact areas between the teeth are clear and there is no overlap of the teeth. The radiograph on the right was taken with film and teeth parallel, but the beam is angled about 20 degrees from the distal. Notice the overlap of the contacts between the teeth. This overlap tends to obscure any caries that may be present. Also notice the root caries on #14 which is apparent in the radiograph on the left, but not in the one on the right.

This is most easily understood using an everyday example. Picture a sharp shadow of your hand with the fingers spread apart. As long as the palm of the hand is perpendicular to the sun or the slide projector, the shadow on the wall gives an accurate representation of the hand with fingers spread. Now imagine slowly twisting your hand so that the palm begins to become parallel to the light coming from the source. Even though you are keeping your fingers spread, the shadow shows the spaces between the fingers progressively getting smaller until the fingers overlap entirely you can no longer discern separate fingers at all.

Splitting the angle:

There is a fairly easily learned technique in which the operator can overcome most of the distortions caused by the impossibility of keeping all three elements (teeth, film and beam) in an ideal relationship. It works especially well to reduce vertical distortions such as elongation and foreshortening in the occlusal/apical length of teeth, but it cannot be used to remove horizontal distortions such as overlapping contacts on bitewing x-rays. This technique is called "splitting the angle", and once mastered, it can speed up the process of taking a full mouth survey. The Rinn type apparatus is not used in the split-angle technique. Instead, the operator uses a Styrofoam Stabe bite block or a non disposable equivalent.

In our everyday analogy, you are standing upright on a flat, horizontal concrete tarmac. As the sun descends in the sky from directly overhead, it eventually will reach an angle at which your shadow on the ground will be exactly as tall as you are. This shadow is not entirely free from distortion, but this is the least distorted shadow that can be achieved when the receptor (the ground) and the object (you) are not parallel to one another. It turns out that this type of image can be produced on an x-ray film by splitting the difference between the angles of the tooth and the film. A simple trick to accomplish this is to use a dual-aiming method. Place the film in the mouth using a Stabe bite block or the film holder from a Rinn apparatus without the ring or the metal rod. Position the film as close to parallel to the long axis of the tooth as is possible.

Now, position the x-ray tube so that it is perpendicular to the film and note the angle of the tube. Call this position 1.
Then reposition the tube so that it is perpendicular to the tooth itself. call this position 2.
Finally, reposition the tube so that it is at an angle that is exactly between position 1 and position 2. This is the angle which will produce the least distorted shadow of the tooth in question.

Note that tube repositioning becomes unnecessary as the operator becomes more familiar with the technique. Once mastered, this technique is actually faster and more accurate than using the Rinn.

One place where this technique becomes essential is with an occlusal film on a child. Occlusal films image the erupted and unerupted incisors. In this technique, the child is told to bite on the film like he would bite on a piece of cardboard, flat between his upper and lower teeth. The film is thus placed in the child's mouth so that it is almost perpendicular to the long axes of both the upper and lower incisors.

Aiming the beam perpendicularly to the film surface would seriously foreshorten the teeth since the teeth are now nearly parallel to the beam. Aiming the beam perpendicularly to the teeth would very seriously elongate them. But aiming at an angle that is midway between perpendicular to the film and perpendicular to the teeth produces a nearly undistorted shadow of the teeth on the film.

Rinn's XCP-system film holders will keep the film perpendicular to the x-ray beam which eliminates one source of distortion, but it cannot eliminate the distortion produced when the film cannot be placed parallel to the teeth. With practice, developing a technique utilizing angle splitting produces less distorted intraoral images, and saves quite a lot of time.

Shadow-Casting Principles

Principle One

Principle Two:

Principle Three:

Principle Four:

Principle Five: